The production of food from carbon dioxide
and water in the presence of chlorophyll utilizing light energy and releasing
oxygen is called photosynthesis. Plants produce food in the form of simple
sugars (carbohydrate). In order for photosynthesis to occur sunlight and
chlorophyll must be present. There are different types of chlorophyll, but the
one involved in photosynthesis is called chlorophyll-A. Chlorophyll is a
pigment which is stored in chloroplasts. During autumn and winter chlorophyll
is not produced allowing the other pigments of brown and red to predominate
giving plants their autumn colors.

PROCEDURE:

Add 1-2 drops of bromothymol blue to
15mL of water in a 25 ml graduated cylinder. Gently blow into the tube using a
straw until it changes color to yellow. Have the students discuss the change.
[The change occurs because CO2 , has been added to the solution,
making it more acidic therefore causing a color change. Remember, humans
produce carbon dioxide and utilize oxygen whereas plants utilize carbon
dioxide and produce oxygen]

Place a piece of elodea in the 25mL
graduated cylinder with the yellow solution and insert a cork stopper in the
beak of the cylinder. Expose the cylinder to sunlight for 20 minutes. [The
yellow solution will change back to blue because the plant will utilize the
carbon dioxide, making the solution less acidic, converting the color back to
blue. ]

In another cylinder, add 1-2 drops of
bromothymol blue to water and gently blow with a straw until the solution
changes color. Plug the top of the cylinder with a stopper and place it in the
sunlight for 20 minutes without elodea. [No change will occur because the
carbon dioxide remains in solution and the acidity of the solution does not
change.]

Prepare another cylinder in the same
manner as the previous one, but do not expose it to sunlight, wait 20 minutes.
[Again, no change will occur because no plant is present and photosynthesis is
not occurring therefore the acidity of the solution remains the same.]

A final cylinder containing an elodea
plant bromothymol blue and water should have been prepared before this
activity and left in a dark place for 24 hours. [No change will occur because
sunlight is required for photosynthesis to occur.] Ask the students to
describe what they observe.

Write the following reaction on the
board:

[carbon dioxide + water - yields -
carbohydrate + oxygen + water]

Explain the exchange of gases and how
humans and plants live together in a mutual relationship. Without plants
humans would not have oxygen and without humans plants would have no carbon
dioxide. Through photosynthesis, plants produce food for both themselves and
other living organisms. Remind students that when animals eat plants they too
are eating the food that the plants produced.